1. Field of the Invention
The present invention relates generally to spray formed rapid tools and, more specifically, to a method of integrating detailed features into a spray formed rapid tool
2. Description of the Related Art
It is known to make a spray formed rapid tool. In spray forming, a master model of a desired tool is produced using a free form fabrication technique. This master model is then used to create a ceramic pattern which is the reverse of the desired tool to be produced. The resulting ceramic pattern is the receptor onto which metal is sprayed to form a deposit in the shape of the desired tool.
Typically, the spray forming process uses a wire-arc spraying. In wire-arc spraying, electric current is carried by two electrically conductive, consumable wires with an electric arc forming between the wire tips. A high-velocity gas jet blowing from behind the consumable wires strips away the molten metal which continuously forms as the wires are melted by the electric arc. The high-velocity gas jet breaks up or atomizes the molten metal into finer particles in order to create a fine distribution of molten metal droplets. The atomizing gas then accelerates the droplets away from the wire tips to the ceramic pattern where the molten metal droplets impact the ceramic pattern to incrementally form a deposit in the shape of the desired tool. The completed desired tool is then mounted and used to produce parts in conventional stamping, die casting, or molding process.
The formation of a totally functional spray formed rapid tool depends upon the incorporation of many subtle design and functional features. These details can be very fine or small as well as embedded on the surface or at some depth in the desired tool.
Finely detailed features which are embedded at some depth require a very thick deposit to properly anchor the features. It is also problematic to spray around deeply embedded features which are "hidden" because voids can result. Formation of voids is due to limitations of the spray and the wide spray pattern of the typical thermal spray process. In addition, thermal spray particles are not self leveling and therefore do not fill voids. These voids may become crack or stress initiation sites of the desired tool.
Detailed features which can cause voids in spray formed rapid tools are grouped into two categories as thin/deep and blind. A thin/deep detailed feature has an opening that is too narrow and thus prevents spray steam particles from penetrating to the bottom of the feature in the ceramic pattern. Typical aspect ratios for a thin/deep detailed feature are greater than 1:1 depth: smallest opening dimension. Common examples of thin/deep detailed features include stiffening ribs and baffles. A thin/deep detailed feature is long and has a depth that is greater than the narrowest width dimension. A blind feature may also be small but it is deep relative to width of its opening. Typically, the aspect ratio between its depth and smallest opening dimension is greater than 1:1. For instance, a feature with a hole 0.25" wide and 0.50" deep has a 2:1 aspect ratio, thus making it difficult to fill with spray stream particles.
Although the above process for making a spray formed rapid tool has worked well, it suffers from the disadvantage that very fine and deeply embedded detailed features must be machined into the spray formed rapid tool after the spray process is completed. This machining process is both laborious and time consuming. Another disadvantage is that the thermal spray coatings are heavily oxidized and the detailed features can only be ground or electro discharge machined (EDM) into the spray formed rapid tool which is time consuming because the electrodes must be as finely detailed as the detailed features and is costly. Therefore, there is a need in the art to integrate detailed features in a spray formed rapid tool that would otherwise need to be machined into the tool.